Dwarf planet

Dwarf planet, body, other than a natural satellite (moon), that orbits the Sun and that is, for practical purposes, smaller than the planet Mercury yet large enough for its own gravity to have rounded its shape substantially. The International Astronomical Union (IAU) adopted this category of solar system bodies in August 2006, designating Pluto, the even more-remote object Eris, and the asteroid Ceres as the first members of the category. Unlike major planets, these bodies are not massive enough to have swept up most smaller nearby bodies by gravitational attraction; they thus failed to grow larger. The IAU agreed to establish a process for determining which other bodies presently known or to be discovered are dwarf planets. In June 2008 the IAU created a new category, plutoids, within the dwarf planet category. Plutoids are dwarf planets that are farther from the Sun than Neptune. All the dwarf planets except Ceres are plutoids; because of its location in the asteroid belt, Ceres is not. For a discussion of the formal conditions set out by the IAU for a body to be a dwarf planet, seeplanet.

Pluto; Charon; Nix; HydraPluto and three of its moons—Charon, Nix, and Hydra—as observed by the Hubble Space Telescope.HST Pluto Companion Search/ESA/NASA